Dr. H. J. Johnston-Lavis—Trachyte at Naples. 77 
but in some cases as much as 125° was obtained. Under high 
powers a large number of minute well-formed cubes about 0-02 
millim. are observable attached all over the surface of these 
hexagonal crystals. What either of these minerals may be is a 
difficult matter to decide, but the striking resemblance of the 
hexagonal one and its analogous mode of occurrence would easily 
lead one to refer it to microsommite. Lastly, but rarely to be met 
with, are minute acicular orange yellow crystals that look at first 
sight like small titanites (semeline). In HCl, however, these dis- 
solve with much effervescence, and the solution gives an abundant 
precipitate with ammonic oxalate, and, therefore, may be referred to 
scalenohedra of calcite. 
The order of crystallization in the cavities seems to have been as 
follows :— 
1, Sanidine; 2, Amphibole; 3, Pyroxene; 4, Rutile?; 5, Sodalite ; 
6, Microsommite ?; 7, Calcite. 
Strangely enough I have not been able to find a trace of mica, 
either in the matrix or in the vesicles. JI am the more struck with 
this, since in some blocks of a very similar rock near Pianura, 
enclosed in the cavities with the amphibole are most beautiful large 
brown plates of biotite. 
The minerals are no doubt sublimates, or more properly result 
from the reaction of different vapours under special conditions of 
temperature and pressure. 
A bulk analysis of this rock was made with special precautions 
as to sampling and the avoidance of regions in which vesicles 
occurred. The results are given below :— 
Si Os BAG ia 57:909 
Tin nO! ae ieo.65. 
Al, Os rite ates 15-786 
Fes O03 eco eco 6°807 
Fe O ee exe 0:010 
Mn O ape sisle 0:230 
Ca O at Am 2:986 
Mg O ale Bais 1-656 
Na, O Bea nae 6:013 
Ke Ohio ee OO 
P2 O; eee eee 0:007 
Cl ite ANG 0-606 
H, O ots ae 0°336 
100:°270 
Sp. Gr. = 2°516 
This analysis gives the composition of a trachyte somewhat similar 
to that of Scarrupata. The uncommon point is the high percentage 
of titanic acid. 
What is remarkable is the very fresh appearance of this rock, 
whilst the minerals of the vesicles seem hardly to be altered, yet 
the bisilicates in the base are in a state of molecular instability, so 
that some doubt exists as to whether these changes are pre- or post- 
eruptive. The great interest of this trachyte will be its relationship 
to two other masses lately met with in its neighbourhood. 
